This invention relates to a method of manufacturing an optical waveguide which is for use in guiding an optical ray and applicable, for example, to an optical switch, an optical coupler, and the like.
As a rule, an optical waveguide of a type described is used for guiding a light wave in an optical device so as to carry out telecommunication with the other devices. The optical waveguide comprises a transparent substrate and a light-transmitting or optical passage buried in the transparent substrate. In this event, the transparent substrate is formed by a material containing a primary ionizable substance, such as sodium (Na), which provides a predetermined refractive index, but the light-transmitting passage is formed by a material containing a secondary ionizable substance, such as silver (Ag), which provides a refractive index greater than the predetermined refractive index.
With this structure, the optical waveguide can guide the light wave through the light-transmitting passage because the light-transmitting passage has a refractive index greater than that of the transparent substrate.
Heretofore, a method is disclosed in Japanese Unexamined Patent Publication No. Syo 48-61157, namely, 61157/1973. In this method, the light-transmitting passage is buried in the transparent substrate by immersing the transparent substrate in an electrolytic solution including silver ions (Ag+) and in another electrolytic solution including sodium ions (Na+). This method may be called a wet method and is effective to completely bury the light-transmitting passage in the transparent substrate. Inasmuch as the light-transmitting passage is completely buried in the transparent substrate, the light-transmitting passage is not exposed to the atmosphere.
However, the wet method is disadvantageous in that the transparent substrate is undesirably eroded by the electrolytic solution during the immersion of the transparent substrate. In addition, it is difficult in the wet method to control the amount of ions which is diffused into the transparent substrate.
Alternatively, another conventional method is described in an article in the Journal of Applied Physics (page 93, 28a-A-2 fall, 1982). This conventional method may be referred to as a dry method and will be described in detail with reference to one figure of the accompanying drawing. At any rate, the light-transmitting passage is formed without using any electrolytic solution Instead, the light-transmitting passage is buried into the transparent substrate by impressing a d.c. voltage in an atmosphere. More particularly, the transparent substrate of, for example, glass is prepared which has primary and secondary surfaces and which contains a primary ionizable substance, for example, Na providing a predetermined refractive index. Thereafter, a diffusion-suppressing layer of, for example, titanium (Ti) is covered on the transparent substrate and is partially removed by the use of a photolithographic technique, to selectively expose the primary surface, and to thereby define an exposed area of the primary surface. The diffusion-suppressing layer may be also called a diffusion-prevention layer. Subsequently, an overlying layer is deposited by the use of sputtering or vacuum evaporation on the exposed area and on the diffusion-suppressing layer. The overlying layer contains a secondary ionizable substance of, for example, Ag providing a refractive index greater than the predetermined refractive index.
Finally, the d.c. voltage is impressed between the overlying layer and the secondary surface of the transparent substrate to ionize the secondary ionizable substance, to diffuse or immigrate secondary ions into the transparent substrate, and to thereby form the light-transmitting passage. Such impression of the d.c. voltage is carried out after electrodes are deposited on the overlying layer and the secondary surface by the use of sputtering or vacuum evaporation.
Next, after the diffusion of the secondary ions, the diffusion-suppressing layer is removed from the transparent substrate together with the overlying layer. Thereafter, an additional layer of a primary ionizable substance, such as chloride, fluoride, or metal is deposited on said primary surface of the transparent substrate by sputtering or vacuum evaporation.
Finally, the d.c. voltage is again impressed between the additional layer and the secondary surface of the transparent substrate to ionize the primary ionizable substance into primary ions. Consequently, the primary ions are diffused or immigrated into the transparent substrate, to thereby form a buried light-transmitting passage.
However, the dry method is disadvantageous in that a difference of stoichiometric compositions is inevitably present between a target and a deposited layer, such as the overlying layer.